Ennaoui cours rabat part III

Photovoltaic Solar Energy Conversion (PVSEC)
‫إﻧﺘﺎج اﻟﻜﻬﺮﺑﺎء ﻣﻦ اﻟﻄﺎﻗﺔ اﻟﺸﻤﺴﻴﺔ‬
Courses on photovoltaic for Moroccan academic staff; 23-27 April, ENIM / Rabat

Organic
Q-Dots

ZnO NRs
PVSEC-Part III
Fundamental and application of Photovoltaic solar
DSSC
cells and system
Ahmed Ennaoui
Helmholtz-Zentrum Berlin für Materialien und Energie
ennaoui@helmholtz-berlin.de
i@h l h lt b li d

Highlight

First
Fi t generation: Silicon
ti Sili
Silicon PV technology
Shockley-Queisser limit
y
Route to high efficiency solar cells
Second Generation: Thin Films
• Substrate Chalcopyrite CIGS vs. Superstrate CdTe solar cells
vs
• Technology: CIGS module processing.
• Thin layer silicon process: a-Si: H / Si
• T d S l cell
Tandem Solar ll

New Concepts for Photovoltaic Energy Conversion
Photoelectrochemical and Dye-sensitized solar cells
Organic solar cells: donor-acceptor hetero-junction
Nanostructures for solar cells: photon management and quantum dots
p g q

Ahmed Ennaoui / Helmholtz-Zentrum Berlin für Materialien und Energie

Silicon the first generation
Copyrighted Material, from internet
Silicon is first choice for solar cells because for good knowledge of Si processing in micro
electronics industry.
Jack Kilby (Texas Instrument)
• Nobel Prize for Physics, 2000
obe e o ys cs, 000
• Co-inventor of the monolithic integrated
circuit (1958) – became the Si microchip.
Moore's law describes a long-term trend in the history of computing hardware: the number of transistors
that
th t can b placed iinexpensively on an iintegrated circuit d bl approximately every t years. N th
be l d i l t t d i it doubles i t l two Now the
Pentium 4 has around 55 million components per chip (2003).
The history of computing hardware is the record of the ongoing effort to make computer hardware
faster, cheaper, and capable of storing more data

1941,
1941 first silicon solar cell was reported
Electronics 38 (8), 114-117 (1965)

Efficiency less than 1%
(
(US Patent 240252, filed 27 March 1941)
, )
Lateral Thinking: Solar cells are optoelectronic devices, they depend on the interaction of electrons, holes,
and photons We need an understanding of semiconductors at the quantum mechanical level.

Brief Business Scenario

Top 10 PV Cell
Producers

Price learn cu e o c ysta e S PV-modules (by
ce ea curve of crystalline Si odu es Cumulative installed PV by 2007
y
doubling the number of total installed PV power drop 1st Germany 3.8 GW
prices by the same factor. 2nd Japan 1.9 GW
3rd US 814 MW
4th Spain 632 MW

Aktuelle Fakten zur Photovoltaik in Deutschland, Fraunhofer ISE / Fassung vom 8.12.2011
Report from Photon International, / http://www.renewableenergyworld.com

First generation: Silicon Solar Cells

SILICON SOLAR PV TECHNOLOGY

Production of Si
Metallurgical Grade Silicon (MG) and Electronic grade (EG-Si),
Metallurgical Grade Silicon (MG) is material with 98-99% purity
Typical impurities (Fe), Al, Ca, Mg)
Produced in about 1 Million tons per year, average price is 2 to 4 $/kg
MG-Si: The sand is heated in a furnace containing a source of carbon
Reduction of SiO2 with C in arc furnace at 1800 oC Heat
MG to Si EG-Si
distillation process with HCl to form SiHCl3)
Fractional distillation (impurity segregation) extremely pure SiHCl3
CVD in a hydrogen atmosphere SiHCl3 into EG-Si Quartz
Crucible

Wafer based Si solar cells
Czochralski (CZ) process.
Float Zone (FZ) Record efficiency solar cells.
FZ is more expensive than Cz material.
Si is not the best: 90% absorption requires >100 µm of Si.
Single Crystals: highest efficiency, slow process, high costs.
Poly (multi) crystalline: low cost, fast process, lower efficiency .

Source: Eicke R. Weber, Fraunhofer-Institute for Solar Energy Systems ISE

Purifying the silicon:
I
STEP 1: Metallurgical Grade Silicon (MG-Silicon is produced from SiO2 melted
and taken through a complex series of reactions in a furnaceV T = 1500 to
Seebeck voltage at Microelectronic
2000 C.
STEP 2: Trichlorosilane (TCS) is created by heating powdered MG-Si at around
300 C in the reactor Imp rities s ch as Fe Al and B are remo ed
reactor, Impurities such Fe, removed. Electronic
t S Grade Chunks
Cold Si + 3HCl SiHCl3 + H2
Hot
STEP 3: TCS is distilled to obtain hyper-pure TCS (<1ppba)d then vaporized,
and
e-
diluted with high-purity hydrogen, and introduced into a deposition reactor to form
l ili
n-type wafer
yp
polysilicon: SiHCl3 + H2→Si + 3HCl
Si
ρ = 2 π s V/I Impurities
Electronic d (EG-Si),
El t i grade (EG Si) 1 ppb I b iti Making single
crucible crystal silicon
STEP 1
Czochralski (CZ) process
Seed crystal slowly grows

STEPE 2 and 3 Device fabrication
1. Surface etch, Texturing Cells
2. Doping: p-n junction formation Ingot sliced
3. Edge etch: removes the junction at the edge to create wafers
4. Oxide Etch: removes oxides formed during diffusion
5. Antireflection coating: Silicon nitride layer reduces reflection
Source: Wacker Chemie AG, Energieverbrauch: etwa 250kWh/kg im TCS-Process, Herstellungspreis von etwa 40-60 €/kg Reinstsilizium


Anti-Reflection Coating
g
Si3N4 layer reduces reflection of sunlight and passivates the cell

.
plasma enhanced chemical vapor deposition (PECVD))


Firing: The metal contacts are heat treated (“fired”) to make contact to the silicon.

Screen Printer with automatic loading and unloading of cells


Firing: The metal contacts are heat treated (“fired”) to make contact to the silicon.

.

Firing furnace to sinter metal contacts

Shockley-Queisser limit Copyrighted Material, from internet
Not all the energy in each absorbed photon can be captured for productive use.
Under AM1 5
U d AM1.5 spectral di t ib ti Single-junction solar cell has a maximal conversion efficiency of ~32%
t l distribution: Si l j ti l ll h i l i ffi i f 32%
Solar Energy Materials & Solar Cells 90, 2329-2337 (2006)
Reflection Loss
1.8% I2R Loss
0.4% 0.4%
%
0.3%

Recombination
1.54% 3.8% Losses
2.0%
1.4% Back Light
Absorption 2.6%

(1) Lattice thermalisation loss (> 50%)
L tti th li ti l
(2) Transparency to photons loss < Band gap
(3) Recombination Loss
(4) Current flow
Source: University of Delaware, USA
(5) Contact voltage loss

Shockley-Queisser limit Copyrighted Material, from internet

Technology approach to high efficiency solar cells
Low reflection Low recombination, High carrier absorption
Thinner emitter, closed spaced metal fingers
Back surface field (p+-p )
Anisotropic texturing (current collection)
Surface Passivation (SiO2 ca 0 01 μm) Key to obtain Voc:
ca. 0,01 m)
Photolithography to have small contact area and high aspect ratio
Laser grooving and electroplating of metal.
TiO2, SiO2, Z S M F2
ZnS, MgF
Technological loss 2N + 1
d ARC = Texturing
nARC 4n ARC Resistive loss
ARC
n2
Top contact
Reflection loss High doping

Recombination
loss ‐ ‐
EBSF

High doping

Traditional cell design

Route to high efficiency solar cells Copyrighted Material, from internet

Traditional cell design MINP PESC IBC PERC PERL
(1) PERL developed at UNSW (EFF. 25%) Passivated Emitter and Rear Locally diffused1
(2) Localized Emitter Cell Using Semiconducting Fingers. (EFF. 18.6%, CZ n-type)
(3) Laser-grooved, buried front contact (LGBC; EFF. 21.1%)

n+
n++ P Buried
contact
(2)
(1)
1 MartinGreen, PIP 2009; 17:183–189, University of New South Wales, Australia
http://www.unsw.edu.au/
(3) Back contact


Thickness of the c-Si absorber without reflectivity and recombination losses
y
⎛ 1 ⎞
η = (1 − R) ⎜1 − e −αW ⎟
⎜ 1 + αL ⎟
⎝ p ⎠
⎡ ⎤
I sc = A . q . ∫ ⎢ η(λ)
{ { . Φ 0 (λ ) . (1 − R λ ) . exp - α λ .d ⎥dλ
123 144 2444 ⎥ 4 3
E G ⎢Collection light
Cell area
⎣ Photon flux Absorbed Light ⎦

The space charge region and tunneling at metals/highly doped semiconductor junction

Highly doped semiconductor
(n++ , p++ = 1020...1021 carriers/cm3) Quantum Mechanics

Tunneling


1.
1 Rsurff Δns ,Δps
Δp
2. Rsurf vns ,vps Nts

1. Reduction of the minority carrier concentration at the Ohmic
y
contact (realized with the back surface field - BSF).
2. Reduction of the Ohmic contact area and reduction of the
surface recombination velocity at the non Ohmic contact
Si – surfaces (realized with contact grids and surface passivation)


What is exactly a p
y passivation?

Most important interface in the world passivating properties observed
in 1960 applied in the world record Si solar cell


BSF: Back Surface Field: The electric field back is to create a potential barrier
(e.g. p+-p junction) on the rear of the cell to ensure passivation.
The potential barrier induced by the difference in doping level between the base and the BSF
tends to confine minority carriers in the base.
These are therefore required to away from the rear face which is characterized
by a very high rate of recombination.
Fabrication tools: Diffusion furnace, PECVD, RTP, Screen-printer, Belt furnace, FZ wafers,
boron BSF
boron-BSF sample, and screen-printing pastes
screen printing

Ag gridlines SiN/SiO2

n+ emitter

Al-Si p-Si
eutectic
BSF
Al/Ag rear
SiN/SiO2 contact

Source: University of Delaware SunPower’s Backside Contact Cell
http://www.sunpowercorp.de/about/ Record efficiency=26.8% at 25W/cm2 Irradiance


Metal Wrap Through
Metal-Wrap-Through Solar Cell

Photovoltech is commercializing the MWT solar cell; efficiencies ~ 15%

Source: University of Delaware


The Sliver® Solar Cell

Origin Energy (Australia) is commercializing the Sliver® Solar Cell (cell efficiencies 20%)

Source: University of Delaware


Rear Interdigitated Single Evaporation-Emitter W Th
R I t di it t d Si l E ti E itt Wrap Through
h

• Both contacts on the rear
• No h d i
N shadowing on the front
th f t
• Carrier collection on two sides
• Rear-side SiO2 passivation
• Laser processing for
ISFH lab result on 10x10 cm2 grooves,
holes and
η = 21%
contact openings
• Single Al evaporation

Source: Institute for Solid State Physics , Leibniz University of Hanover/22nd EU-PVSEC (2007)

Roadmap: Different Generation of Solar cells and PV Power
Costs
First generation
First-generation - based on expensive silicon wafers;
85% of the current commercial market.
Ultimate Second-generation - based on thin films of materials
Thermodynamic
limit
such as amorphous silicon, nanocrystalline silicon,
at 1 sun cadmium telluride, or copper indium selenide. The
materials are less expensive, but research is needed
Shockley- to raise the cells' efficiency.
Queisser limit Third-generation - the research goal: a dramatic
increase in efficiency that maintains the cost
advantage of second-generation materials. Their
design may make use of carrier multiplication, hot
electron extraction, multiple junctions, sunlight
concentration,
concentration or new materials.
materials
Efficiency and cost projections for first-, second- and third- generation photovoltaic technology (wafers, thin-films and
advanced thin-film respectively. The horizontal axis represents the cost of the solar module only; it must be approximately
doubled to include the costs of packaging and mounting. Dotted lines indicate the cost per watt of peak power.
Advanced Research f achieving high efficiency f
for ff from inexpensive materials with so-called third-generation
Concentrating sunlight allows for a greater contribution from multi-photon processes
Stacked cells with different bandgaps capture a greater fraction of the solar spectrum
Carrier multiplication is a quantum-dot phenomenon that results in multiple electron–hole pairs for a single incident photon
Hot electron
Hot-electron extraction provides way to increase the efficiency of nanocrystal-based solar cells by tapping off energetic electrons and
nanocrystal based
holes before they have time to thermally relax.
various thin-film technologies currently being developed reduce the amount (or mass) of light absorbing material required in creating
a solar cell. This can lead to reduced processing costs
Martin Green , Prog. Photovolt: Res. Appl. 9, (2001) pp 123-135

Basic: different ways to make a solar cells / Low cost
processing Thin layer techniques Copyrighted Material, from internet

Physical techniques Chemical techniques Solvent based techniques Electrochemical techniques

Vacuum evaporation Reactive deposition Self-assembling Electroplating
Gel processing Spray methods
Epitaxial deposition Electrophoresis
Chemical vapour deposition Doctor blading
Laser deposition
Langmuir-Blodgett Spin coating
Sputtering
Flow coating
Ionization
Dip coating
Ion-assisted deposition Ionized cluster beam Printing

Flexo printing
Fl i ti Gravure printing
G i ti

Ink jet printing Offset printing

Microcontact printing Relief printing

Screen printing

Kesterite
Ink

Electrophoresis

Spin coating

How do NPs form?
R. Schurr et al. Thin Solid Films 517 (2009) 2465–2468
Projekttreffen NanoPV
A. Ennaoui et al. Thin Solid Films 517 (2009) 2511–251 Kesterite
Vertraulich/Patent pending
A. Ennaoui, Lin, Lux-Steiner PVSEC 2011 Ink

Chemical reaction Critical concentrantion, Aggregation happens
takes place nucleation begins due to its lowering the
free energy

Particles grow and
consume all the solute Hot injection
Best time to synthesize synthesis
nanoparticles

Subsequent growth of the nuclei
lowers the solute concentration

http://www.authorstream.com/Presentation/rahulpupu-976297-nanoparticles/

Nanostructured ZnO From microstructure to nanorodes and fuctionalization
Ennaoui ´Group: Jaison Kavalakkatt, PhD/FU Berlin
Confidential /IP, Patent Pending

Non Vacuum processing / Low Cost Equipments next generation solar cells

Changing electrochemical condition
TE HRTE
M M

5
nm

100 nm

See Concept of Inorganic solid-state nanostructured solar cells
Special issue Ahmed Ennaoui
Solar Energy Materials and Solar Cells, Volume 95, Issue 6, June 2011, Pages 1527-1536

Ahmed Ennaoui / head of a research group: Thin Film and nanostructured solar cells /Solar Energy Division / Helmholtz-Zentrum Berlin für Materialien und Energie

Thin layer silicon process: (a-Si: H / Si) Copyrighted Material, from internet

Heterojunction amorphous silicon / crystalline silicon (a-Si: H / Si)
Si),
say HIT with intrinsic Thin Layer
Two heterojunctions a-Si: H / Si: The "front heterojunction is the" transmitter,
while the second, the rear panel, acts as a field of repulsion or BSF.
, p , p
Intrinsic zone allows "better" surface quality at the junction layer .
transparent conductive oxide (TCO) is deposited to ensure good contact between
the amorphous layer and the metal.
The heterojunction is obtained by depositing technologically "a layer a few “nm”
hydrogenated amorphous silicon, a-Si: H.

Basic: Tandem Cell)

EFF Lab 12 13% / Module 10%
EFF. 12-13%

Back Reflector

Thin film mc Si
mc-Si
Bottom cell

a-Si
Top cell

Textured TCO

Glass substrate

Sun-Light
S Li h

Practical Handbook of Photovoltaics: From Fundamentals to Applications, edited by T. Markvart and L. Castaner. Oxford: Elsevier, 2003

Basic: Efficiencies beyond the Shockley-Queisser limit
Multijunction cells use multiple materials to match the spectrum
spectrum.
The cells are in series; current is passed through device
The current is limited by the layers that produces the least current.
The voltages of the cells add
The higher band gap must see the light first.
By making alloys, all band gaps can be achieved.
Challenge: Lattice matched limited in material combinations GaInP/GaInAs/Ge Cells have powered
Mars Exploration Rovers (MER)
GaInP/GaInAs/Ge Cells record 38.8% @ 240 suns (2005)

New?

(R. King, et al, 20th PVSEC European Conference)

Basic: Efficiencies beyond the Shockley-Queisser limit
Structure of Triple-Junction (3J) Cell

Front Contact
AR Coating
n+ (In)GaAs
n+ AlInP [Si] • Efficiencies up to 41%
n+ I G P [Si]
InGaP InGaP
I G P
p InGaP [Zn] Top Cell
p AlInP [Zn] • Six different elements
p++ AlGaAs [C]
n++ InGaP [Si] Tunnel Junction
n+ AlInP [Si] • Three different dopants
n+ (In)GaAs [Si] InGaAs
p (In)GaAs [Zn] Middle Cell
p+ InGaP [Zn]
p
[ ]
++ AlGaAs [C]
• Practically used:
n++ InGaP [Si] Tunnel Junction 3-junction cells
n+ (In)GaAs [Si]
Buffer Layer
n+ GaAs : 0.1µm
n Ge
G • Research:
p Ge Substrate Bottom Cell
4 to 5 junctions
Back Contact

Yamaguchi et. al., 2003 Space Power Workshop

2nd. Generation: Cu(In,Ga)(S,Se2) Chalcopyrite solar cell
The chalcopyrite structure can be deduced from the
Diamond IV diamond structure according to the Grimm-Sommerfeld-rule,
structure Si which states that a tetragonal structure is formed, if the
average number of valence electrons per atom equals four

nq N + mqM
zincblende structure =4
III-V II-VI n + m + ...
Epitaxial fil
E i i l film: P l lli
Polycrystalline N M elements
N,M
n,m atoms/unit cell
GaAs , InP… thin film: qN, qM valence electrons
CdTe, ZnS

II-IV-V2 I-III-VI2
Epitaxial film:
Polycrystalline thin film:
y y
ZnGeAs,
Z G A …
Cu(In,Ga)(Se,S)2
(Chalcopyrite and related compounds)

I-III-VI2 Alloy: Group I= Cu,
I III VI Cu
Group III= In and Ga,
Group VI = Se and S

Possible combinations of (I, III, VI) elements

⎛Sn⎞ ⎛ Cu ⎞ ⎛ Ga ⎞
(In) ⎜ ⎟
⎜Zn⎟
⎜ ⎟
⎜ Ag ⎟
⎜ ⎟
⎜ In ⎟
⎛S ⎞
⎜ ⎟
⎝ ⎠ ⎜ Au ⎟ ⎜ Al ⎟ ⎜ Se⎟
26 Zn
Z Element ⎝ ⎠ ⎝ ⎠ ⎜Te⎟
1.225 Tetrahedral coordination radius ⎝ ⎠2
Cu(In,Ga)Se2 1.5 Electronegativity
IIIa VIa
3 Li 4 Be 5 B 6 C 7 N 8 O 9 F
2s 0.975 0.853 0.774 0.719 0.678 0.672 2s
2p 0.95 1.5 2.0 2.5 3.0 3.5 3.9
2p
11 Na 12Mg 13 Al 14 Si 15 P 16 S 17 Cl
3s 1.301 1.230 1.173 1.128 1.127 1.127 3s
3p
3 3p
0.9 1.2 Ib IIb 1.5 1.8 2.1 2.5 3.0

3d 19 K 20 Ca 29 Cu 30 Zn 31 Ga 32 Ge 33 As 34 Se 35 Br 3d
4s 1.333 1.225 1.225 1.225 1.225 1.225 1.225 1.225 4s
4p 0.8 1.0 1.8 1.5 1.5 1.8 2.0 2.4 2.8 4p
4d 37 Rb 38 Sr 47Ag 48 Cd 49 In 50 Sn 51 Sb 52 Te 53 I 4d
5s 1.689 1.405 1.405 1.405 1.405 1.405 1.405 1.405 5s
5p 0.8 1.0
1.8 1.5 1.5 1.7 1.8 2.1 2.5 5p
5d 55 Cs 56 Ba 79 Au 80 Hg 81 Tl 82 Pb 83 Bi 84 Po 85 At 5d
6s 1.392 6s
6p 0.75 0.9 2.3 1.8 1.5 1.8 1.8 2.0 2.2 6p

Second Generation: Thin-film Technologies
• Advantage: Low material cost, Reduced mass
• Di d t
Disadvantages: T i materiall (Cd), S
Toxic t i (Cd) Scarce materiall (In, T )
t i (I Te)
• CdTe – 8 – 11% efficiency (18% demonstrated)
• CIGS – 7-11% efficiency (20% demonstrated)

*CIGS based device
CdTe based device
Source: Rommel Noufi, NREL, Colorado, USA,
http://www.nrel.gov/learning/re_photovoltaics.html

Potentials of thin film Cu-chalcopyrite technologies

1. S tt i
1 Sputtering of Cu and In
fC d I
2. Rapid Thermal processing (RTP)

• low material consumtions
• low energy consumption
• hi h productivity l
high d i i large area
• „monolithic“ interconnects - Laser
• new products (e.g. flexible cells)

wafer
f substrate
Wafer based technology Quelle: EI3 Thin film cell structure thickness 1.5-2 µm

Source: HZB / Technology department

Potentials of thin film Cu-chalcopyrite technologies

S
Cu

In

1 kWp : Comparison of c-Si and CuInS2

Module processing

1. KCN etching
2. CBD-Buffer


Technology: Module processing
Monolithic integration for series connection of individual cells
P1: Series of periodic scribes that defines the width of the cells
P2: After the absorber and buffer layer deposition Pulsed Laser
P1
P3: After the window deposition

+Ga +Se
ZnO
Front ZnO of one cell Buffer
connected to the CIGS
back Mo contact of Mo
the next Glass
1. Deposition of Cu, In,Ga
2. RTP/Reaction with S/Se

Source: HZB / EI2 department

Technology: Module processing
Monolithic integration for series connection of individual cells
Loads
-
+
Zn:Al
i-ZnO
CdS
CIGS
+ + + + Mo
Glass
P1 P2 P3

RSC
Laser scribing and mechanical scribing
pulse repetition rate i-ZnO/ZnO:Al
i Z O/Z O Al
pulse power CdS
wavelength and spot diameter
+
Electrical isolation for front and CIGS contact scribes
back
Low series resistance for the interconnect scribe
Mo
Interconnect resistivity as low Glass
as possible
Source: ZSW

Best efficiency from annealing of stacked metal layers

Substrate: soda lime glass coated with Mo Temperature/ C
Temperature/°C
Deposition of Cu and In, Ga layers by sputtering 500-550
Deposition of Se layer by evaporation
Rapid thermal process (RTP) RTP

Advantage: Design of production facilities Time/min
Large-area
Large area deposition Avoidance of toxic H2Se
The most essential factor that decides if the absorber is going to result in a high-
efficiency device, is its Cu content, or the Cu/(Ga+In) ratio

Cu(In.Ga)(S,Se)2

CIGS film should be slightly Cu deficient with a thin even more Cu deficient surface
Cu-deficient, thin, Cu-deficient
layer. This surface layer corresponds to the stable ordered vacancy (OVC) Cu(In,Ga)3Se5.

Fundamental understanding

ZnO

Absorber

Fundamental understanding

buffer CIS EC
ZnO
EC < EC ?
ZnS at
EV
Absorber
The GBs

Zn CIS, CIGS
AO
l
Buffer
Barrier for
recombination:

Absorber

Material Properties: Phases Diagram
Simplified version of the ternary phase diagram
Reduced to pseudo-binary phase diagram along the red dashed line
Bold blue line: photovoltaic-quality material
Relevant phases: α-, β-, γ- , δ-phase and Cu2Se
α β γ δ phase,and

CuIn3S5
Not
found

α: chalcopyrite CuInSe2
β: defect chalcopyrite Cu(In,Ga)3Se5
γ: Cu(In,Ga)5Se8

Material Properties: Phases Diagram
α phase
α-phase (CuInSe2):
• Optimal range for efficient thin film solar cells: 22-24 at %
• α-phase highly narrowed @RT
• Possible at growth temp.: 500-550°C, @RT: phase separation into α+β
500 550 C, α β

β phase
β-phase (CuIn3Se5)
• built by ordered arrays of defect pairs
• anti sites (VCu, InCu)

δ-phase (high-temperature phase)
• built by disordering Cu & In sub-lattice

Cu2Se
• built from chalcopyrite structure by
• Cu interstitials Cui & CuIn anti sites

Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004.

Material Properties: Impurities & Defects
Partial replacement of In with Ga; 20-30% of In replaced: Ga/(Ga+In) ~ 0.3
20 30% Ga/(Ga In)
Band gap adjustment: 1.03eV-1.7 eV
- Widening of bandgap at the surface of the
Incorporation of 0.1 at % Na film
Na (Se) (stability d
N 2(S )1+n ( t bilit decrease with n↑)
ith ↑) - The surface composition of Cu-poor CIGS
Cu poor
Better film morphology films
Passivation of grain-boundaries (Ga+In)/(Ga +In+Cu) ca. 0.75
Higher p yp conductivity
g p-type y - The bulk compositions
Reduce defect concentration 0.5< (Ga+In)= (Ga+In+Cu) < 0.75.
The are many defect
- 3 vacancies: VCu, VGa, VSe.
- 3 i t titi l Cui, G i, S i.
interstitials: C Ga Se
Phase segregation of Cu(In,Ga)3Se5
- 6 antisites: occurs at the surface of the films.
CuGa, CuSe, GaCu, GaSe, SeCu, SeGa

Ordered-Vacancy/ Defect Compounds (OVC/ODC)
Ordered or disordered arrays of vacancies are occupying the cation sites
They can exceed the local range of the unit cell, we called vacancy compounds
Superlattice structures of the ideal chalcopyrite, reported as stable phases: OVC/ODC
OVC/ODC are observed in slightly Cu-deficient: Cu(In,Ga)3Se5
Schock, Rommel Noufi, , Prog. Photovolt. Res. Appl. 8, (2000) pp. 151-160

Roll-to-Roll deposition (R2R)
Ion beam supported low temperature Source: Fahoum Mounir/Habilitation
deposition of Cu, In, Ga, Se
fC G S
Substrate:
Mo coated polyimide/ stainless steel foil
(F f
Fe from th substrate?)
the b t t ?)
Alternative Electrochemistry
Advantages:
• Low cost production
• Flexible modules
• High power per weight ratio

Voltag
e
- +

In,Ga,Cu -ions
, , Annealing Buffer TCO
G C In, Se
Ga,Cu, I S

Recombination mechanism issue

Ea nkT ⎛ j00 ⎞
VOC = − ln⎜
⎜ j
⎟
⎟
q q ⎝ SC ⎠
A: Diode quality factor
EA: Activation energy
J00 : Prefactor, weakly temperature-dependent
Cu(In,Ga)Se2
EC
Buffer
B ff
(1): interface recombination Eg
2
EF
Ea = Φ b 1 EV
Φb
(2): bulk recombination
E a = Eg

Important Remarks
Conversion efficiencies achieved by CuInS2 (EG
y (
= 1.53 eV) or CuGaSe2 (EG = 1.7 eV) absorbers
are considerably lower than those achieved by Burried pn-junction
low band gap Cu(In,Ga)Se2 or even CuInSe2. OVC
Cu(In Ga)Se p
p-Cu(In,Ga)Se2
( , )

Why? OVC

In l b d
I low band gap Cu(In,Ga)Se2
C (I G )S
•Formation of weakly n-type OVC layer
•The bulk is p-type
p yp
•Buried p-n junction n ΔEV
n-Cu(In,Ga)3Se5

OVC minimizes the recombination at the CIGS/buffer interface.
OVC surface layer has direct and wider band gap than the bulk
Φ
OVC increases further the barrier ,Φ, for recombination at CIGS/CdS
That is the key to high-efficiency solar cells.

Third Generation: Multi-junction Cells
Multi-
Copyrighted Material, HZB

Technology: CIGS module processing

N. Naghavi, D. Abou-Ras, N. Allsop, N. Barreau, S. Bu¨ cheler, A. Ennaoui, C.-H. Fischer, C. Guillen, D.
Hariskos, J. Herrero, R. Klenk, K. Kushiya, D. Lincot, R. Menner, T. Nakada, C. Platzer-Björkman, S.
Spiering, A.N. Tiwari and T. Törndahl.
Prog. Photovolt: Res. Appl. (
g pp (2010). Published online in Wiley InterScience, Vol. 18, issue 6 (2011) pp. 411-
) y , , ( ) pp
433

The world record chalcopyrite solar cell

Cu(In,Ga)Se2

New Concepts for Photovoltaic Energy Conversion
(Photo)electrochemical and Dye-sensitized solar cells
Organic solar cells: donor-acceptor hetero-junction
Nanostructures for solar cells

Semiconductor/Liquid versus Semiconductor/Metal Junction

Vacuum level
0
Φ χ qχ

CB
qΦΜ
CB
EF,SC
EF,SC qVB
H+/H2 qVBB
Metal
EC
0 EC CE Back EF,SC EF,Metal
EF,SC EF,redox contact
Back
H2O/H2 contact
VB - 4.5 eV VB
1.23V
Semiconductor (WE) Redox
SCE EV Electrolyte EV
e.g. I-/I2 Metal
+0.243V Semiconductor
Semiconductor e.g. Si e.g. Au
V vs. NHE e.g. TiO2

Electrochemical scale Solid state scale
Summer Semester Osaka University-Japan for graduate student in Research Center for Solar Energy Chemistry/Courses: Photovoltaic and hydrogen Research and development R&D

Semiconductor/Liquid versus Semiconductor/Metal Junction

Summer Semester Osaka University-Japan for graduate student in Research Center for Solar Energy Chemistry/Courses: Photovoltaic and hydrogen Research and development R&D

Photoelectrochemical Solar Cell (PECs): Photovoltaic mode

‐
Reduction

Sc ‐M Back
contact I2 + e‐
+ Countre
I‐ + h+
Electrode
(CE)

Oxidation

I‐ ‐+ h+ + I2 + e‐ ‐
I + h I2 + e
Electron and holes are photogenerated
Holes are moved to the surface of the WE
-- current
react with I I‐ + h+
Electron are moved to the back contact
V
reacts with I2 i th other side (CE)
t ith in the th id
Voltage vs. redox
I2 + e‐
Source: A.J. Nozik, National Renewable Energy Laboratory

Solar cells that mimic plants

p y
Chlorophyll Light absorption Dye
y

Charge transfer protein e- transfer Semiconductor oxide (TiO2)

Proton pump Hole transfer Electrolyte


Solar cells that mimic plants: DSSC

HOMO

LUMO

CO2
Sugar
H2O
O2
Photosynthesis
The most widely used sensitizer abbreviated as N3.
y
“cis-Ru(SCN)2L2 (L = 2,2'-bipyridyl-4,4'-dicarboxylate)”

source: partly http://en.wikipedia.org/wiki/Dye-sensitized_solar_cell
Grätzel, M., Journal of Photochemistry and Photobiology C: Photochemistry Reviews 2003, 4, 145


HOMO: highest occupied molecular orbital

LUMO: lowest unoccupied molecular orbital

HOMO

LUMO

CO2
Sugar
H2O
O2
Photosynthesis
The most widely used sensitizer abbreviated as N3.
y
“cis-Ru(SCN)2L2 (L = 2,2'-bipyridyl-4,4'-dicarboxylate)”

source: partly http://en.wikipedia.org/wiki/Dye-sensitized_solar_cell
Grätzel, M., Journal of Photochemistry and Photobiology C: Photochemistry Reviews 2003, 4, 145

Solar cells that mimic plants Copyrighted Material, from internet

Few simple materials and you can create your own Grätzel Cell
The most widely used sensitizer abbreviated as N3. “cis-Ru(SCN)2L2 (L = 2,2'-bipyridyl-4,4'-dicarboxylate)”

Ru(II) + hν → Ru(II)*

Ru(II)* → Ru(III) + e-

I3- + 2e-→ 3I-

3I- + Ru(III)→ I3- + Ru (II)

I‐ + h+
DSSC
Module

I2 + e‐

Solar cells that mimic plants Copyrighted Material, from internet

Generation Transport Back
B k reaction ( ) with I3-
ti (c) ith
∂n ∂n 2 ( n − n0 ) τn = 1/kcb [I3-]
= α Ie −α x + Dn 2 −
Ru(III)/Ru*(II)) ∂t ∂x τn

( )
(b)
(c) (a)
Ru*(II)/Ru(II))


http://www.solaronix.com/

Mesoporous TiO2 anatase

Efficiency of 10 % was obtained by the solar cells assembled at the EPFL in Lausanne
(simulated sunlight AM 1.5, 1000 W/m2) Eff. = 10 %, AM 1.5, VOC = 823 mV, ISC = 16.9 mA/cm2, FF = 72.5 %)
Download Dye Solar Cells Assembly Instructions @ : http://www.solaronix.com/technology/assembly/

Nanocrystalline based Solar cells
Electron holes photogenerated
Immediately injected in mesoporous TiO2 (or ZnO NRs)

ZnO
nanorodes

T. Dittrich, A. Belaidi, A. Ennaoui J B Sambur et al. Science 2010;330:63-66
Extremely Thin Absorber Band energy diagram indicating the relevant energy levels
and kinetic processes that describe PbS QD ET and HT into
Concept of Inorganic solid-state nanostructured solar cells
the TiO2 conduction band and the sulfide/polysulfide
Solar Energy Materials and Solar Cells, Volume 95, Issue 6, June
electrolyte, respectively.
2011, Pages 1527-1536

Photoelectrochemical solar cells (PECs) Photoelectrolysis mode
1/C2
Band gap must V+v(t)
be at least 1 8 2 0 eV
1.8-2.0
V
V But small enough to absorb most sunlight
Lock‐in
Material requirements
b
Potentiostat
v=vme edges must straddle Redox potentials
Band iωt

Fast charge transfer WE RE CE

Determination of Flat Band PotentialStable in aqueous solution
(Vfb)

I
hν>EG
V

EC Metal WE RE CE

Back EF,SC EF,redox
(CE)
contact
1.23eV
1.23eV

EV Electrolyte
(
(WE)
) Anode: 2H20 4e- + 02 + 4H+
E
Cathode: 4H20 + 4e- 4OH- + 2H2

A. Ennaoui and et al. Solar Energy materials and Solar Cells Volume 29 (1993), Pages 289-370
This lecture was presented @ Osaka University-Japan for graduate student in Research Center for Solar Energy Chemistry/Courses: Photovoltaic and hydrogen Research and development R&D

Determination of Flat Band Potential (Vfb)

V+v(t)
Lock‐in Potentiostat
v=vmeiωt
WE RE CE

vacuum
0

H+/H2

Ref.

A. Ennaoui and et al. Solar Energy materials and Solar Cells Volume 29 (1993), Pages 289-370
This lecture was presented @ Osaka University-Japan for graduate student in Research Center for Solar Energy Chemistry/Courses: Photovoltaic and hydrogen Research and development R&D

Materials suitable for solar PECs

Photoelectrochemical solar cells (PECs) Photoelectrolysis mode

D
D
D D
D
D
D

H2O→2H2+O2 ∆V=1.23V, ∆G=238kJ/mol

Source: Mildred Dresselhaus, Massachusetts Institute of Technology

d0 and d10 metal oxides

GaN-ZnO (Ga1-xZnx)-(N1-xOx)

d0 d10
Ti4+: TiO2, SrTiO3, K2La2Ti3O10
: TiO SrTiO K Ga3+: ZnGa2O4
3 : ZnGa

Zr4+: ZrO2 In3+: AInO2 (A=Li, Na)
Nb5+: K4Nb6O17, Sr2Nb2O7 Ge4+: Zn2GeO4
Ta5+: ATaO3(A=Li, Na, K), BaTa2O6 Sn4+: Sr2SnO4
W6 : AMWO6 (
6+ (A=Rb, Cs; M=Nb, Ta)
b b ) Sb5+: NaSbO7
N replaces O in certain positions, providing a smaller band gap.
Problems with getting the nitrogen there without too many defects.
Oxygen free options: Ta3N5, G 3N4
O f ti T Ge

Domen et al. New Non‐Oxide Photocatalysts Designed for Overall Water Splitting under Visible Light. J. Phys. Chem. 2007

Use of PV for H2 production
Hydrogen and Oxygen are p
y g yg produced using p
g photovoltaic effect

Test of security
- No damage to hydrogen car
- Gasoline car completely destroyed
p n p n p n
Solid state solar
cells

O2 H2

e- e-

H2 H
+
O
Dark electrolysis cell

Source: Partly A.J. Nozik, National Renewable Energy Laboratory

Water splitting: Hydrogen production
Challenge: Material requirement :
Material/catalysts, nano-materials, membranes (need Brainstorming )
Understand and control the interaction of hydrogen with materials

H2O→2H2+O2 ∆V=1.23V, ∆G=238kJ/mol
Source: Mildred Dresselhaus, Massachusetts Institute of Technology millie@mgm.mit.edu

Fuel Cells Copyrighted Material, from internet

Fuel Cell uses a constant flow of
H2 to produce energy.
Catalyst = Pt Very expensive
Reactionthe Pt quantity between
Minimize
takes place
q y
H2 and Othe active layer structure
Improve 2 electrical energy.
The most common fuel cell uses
Propose new materials
Proton Exchange Membrane, o PEM
oto c a ge e b a e, or
Need of catalyst (e.g. platinum
for a reaction that ionizes the gas
O2 is ionized to O2- 2

H2 is ionized to 2H+
2H+ + O2- = H2O
O2- and H+ combine
Energy is given off in The “waste products” are water and heat
electron form and gives
off power to run an engine

Advantages and Challenges Copyrighted Material, from internet

Advantages
Zero emission
No dependence on foreign oil
Ability to harvest solar and renewable energy
Abilit t h t l d bl
Not many moving part in a car
Hydrogen weighs less than g
y g g gasoline
car would not need as much energy to move

Challenges
g
Still expensive to equip a car with a hydrogen fuel cell.
Hydrogen is expensive to make, store, and transport
The center is a platinum plate which is very expensive
National Program in USA since 2007:
1 billion dollars to date in hydrogen car research for the “develop
hydrogen, fuel cell and infrastructure technologies to make fuel-cell
vehicles practical and cost-effective by 2020.”

Basic: Brief Business Scenario Copyrighted Material, from internet

1999 FOUNDED, 2001 BEGAN WITH THE PRODUCTION OF SILICON SOLAR CELLS
WITH 19 EMPLOYEES.

BY 2009, 2,600 EMPLOYEES (2007, 1700 EMPLOYEES)

NOW THE LARGEST SOLAR CELL MANUFACTURER IN THE WORLD. (SINCE 2007)
WORLD
CONTINUE TO EXPAND PRODUCTION IN BITTERFELD-WOLFEN, GERMANY AND
START
CONSTRUCTION OF NEW MALAYSIAN PRODUCTION FACILITY.
ALONGSIDE THE MONO-CRYSTALLINE AND POLYCRYSTALLINE (90% OF
BUSINESS) CORE
BUSINESS, WE USE A WIDE RANGE OF TECHNOLOGIES TO DEVELOP AND
PRODUCE
THIN-FILM MODULES. (THIN-FILM - 25% SHARE OF SMALLER MARKET)
Year over year, Q-Cells SE has been able to grow revenues from €790.4M to €1.4B.

http://investing.businessweek.com

Basic: Brief Business Scenario

SunTech Power (China)
- THE COMPANY WAS FOUNDED IN 2001 BY ZHENGRONG SHI
- SALES $1.9B 2008, 1.3B 2007 PROFITABLE
- EMPLOYEES: 6784
- WORLDS LARGEST SILICON CELL MAKER
- AVERAGE CONVERSION EFFICIENCY RATES OF THEIR
- MONOCRYSTALLINE AND MULTICRYSTALLINE SILICON PV CELLS
- 16.4% AND 14.9% RESPECTIVELY
- 2009 ANNOUNCES PLAN TO BUILD MANUFACTURING PLANT IN US

Zhengrong Shi Boen in 1963 in
China, finished his Master in
China then he went to University
of New South Wales (Austria). He
( ) 130KW
obtained his doctorate degree on 8MW China
43KW 0.092-0.3-3.8MW 3MW
Nevada
solar power technology and Spanien Germany China
returned to China in 2001 to set
14MW 5.1MW 10 MW
up his solar power company (Net Spanien
Nevada Abu Dhabi
worth US$2.9 Billion (2008)

48KW
Australien
500KW
Nevada

http://eu.suntech-power.com


Capacity 130MW

Expansion
70MW
55MW

30MW
20MW
R&D

1980 1999 2006 2007 2008 2010
Kaneka has been specializing in thin-film silicon technology:
1980 Started study of a-Si technology
1987 Participated in NEDO project (Government funded R&D)
1999 Started 20MW/yr commercial production
2006 Announced capacity expansion:
- up to 30MW in 2006, 55MW in 2007, 70MW in 2008
2007 Introduction of new Hybrid PV
Announced capacity expansion: up to 130MW in 2010

Excitonic solar cells Copyrighted Material, from internet

Exciton
LUMO electrons

holes
Interface HOMO

• all organic: polymer and/or molecular
• hybrid organic/inorganic
•ddye-sensitized cell
iti d ll

Donor acceptor concept Copyrighted Material, from internet

Donor acceptor concept Copyrighted Material, from internet

Interpenetrating Nanostructured Networks

η record = 4,8%
η FMF, ISE = 3,7%

Aluminum
Absorber
Akzeptor Polymer Anode
ITO
Substrate

Donor

The light falls on the polymer
Electron/hole is
El t /h l i generated t d
The electron is captured C60

The biggest Challenge Copyrighted Material, from internet

Reducing the cost/watt of delivered solar electricity
Find a concepts for a more efficient PV systems
More efficiency, More abundant materials, Non-toxic material, Durability
First Generation
Fi t G ti
Crystalline Si will remain the dominant PV technology for a long time,
the current shortage will be overcome by increased production of pure Si
and the introduction of purified metallurgical-grade Si.
p g g

Second Generation
Thin film modules out of a-Si, CIS, or CdTe have an interesting market opportunity today, their long-term
success will depend on efficiency improvements and cost reduction
reduction.

Third Generation
TANDEM CELLS: Because sunlight is made up of many colours of different energy, from the high energy
ultraviolet to the low energy infrared, a combination of solar cells of different materials can convert
sunlight more efficiently than any single cell

Multiple Exciton Generation: The objective is fighting termalization: In quantum dots, the rate of energy
dots
dissipation is significantly reduced and one photon creates more than one exciton via impact ionization
Higher photocurrent via impact ionization (inverse Auger process)

Thank you so much

Questions or comments?

PVSEC 23th – 27th. 2012 / Rabat - Morocco
Prof. Dr. Ahmed Ennaoui
Helmholtz-Zentrum Berlin für Materialien und Energie

Parking: produce electricity and have the shadow

Ennaoui cours rabat part III

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